The vulcanization of diene elastomers by sulphur is widely used in the rubber industry, in particular in the tire industry. The principle of vulcanization lies in the creation of sulphur bridges between two macromolecules by reaction with the double bonds of these diene elastomers.
Use is made, to vulcanize diene elastomers, of a relatively complex vulcanization system comprising, in addition to sulphur, a primary vulcanization accelerator, such as sulphenamides comprising a benzothiazole ring system, and various secondary vulcanization accelerators or vulcanization activators, very particularly zinc derivatives, such as zinc oxide (ZnO), alone or used with fatty acids.
The sulphenamides comprising a benzothiazole ring system used as primary vulcanization accelerators are, for example, N-cyclohexyl-2-benzothiazolesulphenamide (abbreviated to “CBS”), N,N-dicyclohexyl-2-benzothiazolesulphenamide (abbreviated to “DCBS”), N-tert-butyl-2-benzothiazolesulphenamide (abbreviated to “TBBS”) and the mixtures of these compounds.
However, vulcanization with sulphur has the known disadvantage of resulting in limited resistance of the vulcanizates obtained due to the thermal ageing of the latter. In particular, the vulcanizates of diene elastomers crosslinked starting from sulphur exhibit high sensitivity to the temperature when the latter reaches a value in the vicinity of the initial curing or vulcanization temperature. This results in a fall in the density of the sulphur bridges initially formed during the vulcanization, the distribution of the vulcanization network changing in the direction of shortening, that is to say a decrease in the polysulphide bridges to the advantage of the monosulphide bridges. This phenomenon, known under the term of reversion, is accompanied by deterioration in the mechanical properties of the vulcanizates.